Chemistry HL's Sample Internal Assessment
How does the exposure to UV light (wavelength of 200nm) for different lengths of time (0mins, 15mins, 30mins, 45mins, 60mins) affect the concentration of Vitamin C (ascorbic acid) measured through iodine titration
Table of content
Exploration
Introduction
Ascorbic acid, also called vitamin C, is widely known for protecting the body cells from free radicals, playing a role in preventing cancer, chronic heart diseases, and a ray of other diseases. (Chambial et al, 2013) Vitamin C has been used in skin care products since the 1970s. There are multiple benefits of ascorbic acid on the skin, it provides antioxidant properties that reduce the destructive properties of free radicals naturally emitted by the sun. It also promotes collagen synthesis by acting as a catalyst, providing the skin with better elasticity and reducing fine lines. (Doyle, 2023)
The use of vitamin C in skin care products was widely available after Dr. Sheldon Pinnell’s pioneering the formulation of L-ascorbic acid, which is the most stable and active form of vitamin C to retain its effectiveness. He also demonstrated antioxidant properties and its ability to neutralize free radicals. (SkinCeuticals, n.d.) This research led to more use of vitamin C in skin care products.
With the increasing demand for beauty standards and physical aesthetics, the properties of vitamin C help with radiant skin that boosts the self-confidence of individuals. It enhances the positive personal perception and in turn, provides the individual with more positive mental well-being. While healthy skin is not only a matter of physical aesthetics, it improves one's overall health. As mentioned before, ascorbic acid protects the skin from environmental stressors, preventing skin diseases. This reduces the requirement of healthcare costs and improves the quality of life. (Piperberry, 2020)
However, the benefits of vitamin C skin care are still limited to the potential exposure to UV light, which is naturally emitted by the sun’s radiation. The effectiveness of vitamin C is vulnerable during the storage conditions and transportation of the products. The ascorbic acid products may be affected under the sun due to UV rays, making up around 10% of the sunlight exposed to the Earth’s surface. (Lucas, 2017) UV radiation is high-energy radiation emitted at a wavelength of 100-400 nm. (WHO, 2016)
Therefore, this study aims to investigate the effect of direct exposure to UV light on ascorbic acid degradation. A control group of 0 seconds was established and the period of exposure to 200 nm of UV light was set at 900, 1800, 2700 and 3600 at 900 seconds (15-minute) intervals. The final concentration of ascorbic acid will reflect the degradation of ascorbic acid over time.
Investigation
Background information
Ascorbic acid is one of the numerous essential vitamins that the body requires. It is found in natural sources of fruits and vegetables and it is water soluble. It has a formula of C6 H8 O6 . The human body cannot naturally synthesise vitamin C, therefore the ingestion of external sources of vitamin C from supplements or food is required to maintain a constant level of vitamin C in the body for biological processes. (National Institutes for Health, n.d.)
The properties of ascorbic acid allow it to act as an antioxidant. It can act as a reducing agent that undergoes oxidative reaction to produce ascorbate. (FoodCrumbles, 2019) When ascorbic acid is exposed to UV light, it undergoes photodegradation, where it undergoes further one electron oxidative reactions producing ascorbyl free radicals. (Aguilar et al, 2019)
When exposed to UV light, ascorbic acid molecules absorb energy from UV photons. The energy causes the ascorbic acid electrons to move to higher energy levels. Under the excited state of electrons cause the covalent bonds to be broken down, resulting in the formation of free radicals ascorbyl. The free radicals are unstable, making it highly reactive due to the presence of unpaired electrons. The presence of free radicals in the body may lead to oxidative damage. (Dix, 2018) This degradation of ascorbic acid results in ascorbic acid breaking down into smaller compounds that do not hold the same nutrients as the original compound.
To investigate the degradation of ascorbic acid under the exposure of UV light. A redox iodine titration was selected to calculate the final concentration of ascorbic acid after UV light exposure. In the titration, iodine (I2) solution is the titrant, and starch ((C6 H10 O5)n) acts as the indicator signifying the endpoint of the titration. Iodine reacts with starch to form a dark blue colour from brown, the endpoint of the titration is reached when the analyte turns into a permanent dark blue colour. (Tucker, n.d.) Iodine reacts with ascorbic acid shown in this equation: C6 H8 O6 + I2 → 2I−+C6 H6 O6 + 2H+
The starch solution is added to the ascorbic acid solution, the solution is titrated against the iodine solution. The excess iodine reacts with starch after ascorbic acid is oxidized. This forms a dark blue colour indicating the end point of the titration.
Research question
“How does the exposure to UV light (wavelength of 200nm) for different lengths of time (0mins, 15mins, 30mins, 45mins, 60mins) affect the concentration of Vitamin C (ascorbic acid) measured through iodine titration ”
Variables
Independent variable - The length of time of direct exposure of ascorbic acid solution (conc) to ultraviolet light (200nm) for 0, 900, 1800, 2700, 3600 seconds (± 1s)
Dependent variable - The concentration of ascorbic acid solution (mol dm −3) after exposure to ultraviolet light measured by iodine solution titration
Controlled Variables
| Variable | Method of Control | Significance |
|---|---|---|
Mass of ascorbic acid powder (0.176g ±0.001g) | Mass of the ascorbic acid powder was carefully measured using an electronic weighing scale as close to the desired mass. | The mass of ascorbic acid is crucial to ensure all the concentrations of the samples prepared have minimal differences. This ensures accurate results for the analysis of the effect of UV light. |
Volume of ascorbic acid solution (5cm3 ± 0.1cm3) | The volume of ascorbic acid solution is carefully measured with a measuring cylinder at eye level. | The volume of ascorbic acid is controlled to ensure minimal differences. Also to ensure data collection is possible to calculate and analyze the results. |
Mass of sodium thiosulfate powder (0.158g ±0.001g) | Mass of the sodium thiosulfate powder was carefully measured using an electronic weighing scale as close to the desired mass. | The mass of sodium thiosulfate powder is controlled as it is used to standardize the iodine solution. The mass is crucial to determine the concentration of iodine solution to achieve accurate data analysis. |
| The time of exposure to ultraviolet light | A timer was set and closely monitored to ensure the length of time of UV light | The time of exposure was determined from preliminary tests. Any extra length of time of |
(900, 1800, 2700, 3600 seconds ±1s) | exposure does not exceed the desired value. | exposure may cause varying results. Therefore to ensure minimal difference of the samples exposed to UV light also ensures similar results. |
The wavelength of ultraviolet light (200nm) | All samples of ascorbic acid solution are exposed under the same “Seven Master UV sterilizer Submersible 5W 200nm” UV light | Under the exposure of the same lamp for all samples minimizes the difference of the intensity of the wavelengths of the UV light. A sterilizer lamp was chosen as it provided enough radiation that imitate the wavelength of the sun. (WHO, 2016) It provides equal and optimal light exposure across the surface of the experiment. It is also not too intense to cause major damage to the atmosphere. |
Figure 5 - Table On Controlled Variables
Uncontrolled variable -
Temperature of the laboratory varying around 23°C is not controlled. The temperature fluctuations should have minimal effect on the results as no windows were opened and the air conditioning was consistent throughout the experiment.
Materials and equipment
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Figure 6 - Table On Materials And Equipment
Procedure
Preparation of (0.004 mol dm-3) sodium thiosulfate solution for standardisation
- Weight 0.158g of sodium thiosulfate powder using an electronic weighing scale.
- Put the sodium thiosulfate powder into a 500 cm3 beaker.
- Add 250 cm3 of distilled water into the same beaker.
- Using a clean glass rod, stir the solution until no white powder can be seen.
- Transfer the solution into an airtight glass bottle to prevent deterioration.
Preparation of (0.01 mol dm-3) Ascorbic Acid Solution
- Weight 0.176g of ascorbic acid powder using an electronic weighing scale
- Put the ascorbic acid powder into 100 cm3 beaker
- Add 100 cm3 of distilled water into the same beaker
- Using a clean glass rod, stir the solution until all white powder dissolves in water.
- Transfer the solution into an airtight glass bottle in a dark area avoiding direct light sources to prevent oxidation of the solution.
Set Up of UV light apparatus
- Prepare a cardboard box with a lid with a height of more than 25 cm3 , a width of 20, cm3 and a length of 35cm3
- Cut out an opening on the box lid 15cm × 5cm (depending on the size of the UV light)
- Put the UV light on the opening of the lid and tape it down to the lid
- Ensure that when covering the lid over the box, the inside should be completely dark
- Put the beaker containing 0.01 mol dm-3 of 10 cm3 ascorbic acid solution in the box
- Cover the lid of the box with the UV light
- Switch on the UV light and start the timer for 900 seconds
- Switch off the UV light after 900 seconds
- Remove the beaker of ascorbic acid solution and immediately conduct a titration experiment
- Repeat step 5 to 9 for the different length of time (900, 1800, 2700, 3600 seconds)
Iodine solution titration procedure
- Fill the burette with 0.003 mol dm-3 iodine solution, the initial volume was recorded on paper
- After 0 seconds, measure 5cm3 of the 0.01 mol dm-3 ascorbic acid solution using a 10cm3 measuring cylinder and add it to a 50cm3 conical flask
- Measure 5cm3 of distilled water and add it to the same conical flask
- Using a graduated pipette, measure 1cm3 of 1% starch solution and add it to the conical flask
- Put a magnetic stirrer into the conical flask on a hot plate and turn on the stirrer to ensure the mixture will be mixed
- Titrate the solution until the endpoint is reached displaying a permanent trace of a dark blue hue
- Record the final volume of iodine solution on paper
- Repeat steps 2 to 7 for each exposure time (900, 1800, 2700, 3600 seconds)
Safety precautions
Ultraviolet Light | Ultraviolet light is dangerous and damaging to the eyes and skin. Therefore, lab coats and safety goggles should be worn throughout the procedure. The conductor of the experiment should not look directly at the UV light source. |
Sodium Thiosulfate | Sodium thiosulfate is hazardous causing skin or eye irritation and burns. Lab |
Solution | coats, latex gloves and safety goggles should be worn throughout the experiment to prevent direct exposure to the solution. |
Iodine Solution | An iodine solution may stain clothing and may cause allergy in some individuals. Lab coats, safety goggles and latex gloves should be worn to minimise contact with the solution. |
Figure 7 - Table On Safety Precautions
Environmental impacts and ethics
Quantitative Observations
Raw Data
- Refer to Appendix A for the 5 trials of the volume (cm3 ± 0.05) of 0.003 mol dm−3 iodine solution Used to Titrate Ascorbic Acid Solution Until the End Point After Exposure to UV Light For 0, 900, 1800, 2700 and 3600 seconds (±1s) (cm3)
Qualitative observations
- The colour and smell of the ascorbic acid solution did not change as the time of UV light exposure increases
- No change in colour is observed after the addition of starch solution
- A faint blue is observed for a short time when the iodine solution is dropped into the ascorbic acid starch solution
- The solution turns dark blue after the ascorbic acid starch solution is titrated against the iodine solution after it reaches the endpoint
Processed Data
Standardisation of Iodine with Sodium Thiosulfate Solution
Figure 9 - Table On The Volume of Sodium Thiosulfate Solution Used to Standardise Iodine Solution (cm3) ( ± 0. 05cm3)
Calculation of lodine solution concentration
- The reaction of Iodine with Sodium Thiosulfate solution equation
\(I_2 + 2S_2 O_3^{-2} → S_4 O_6^{ −2} + 2I^{-}\)
- Concentration of sodium thiosulfate used
\(number \ \ of \ \ moles =\frac{mass}{molar \ mass}\)
\(n(Na_2S_2O_3) =\frac{0.158g}{158.11\ g\ mol^-}=9.99× 10^{-4} mol\ \ dm^{-3}\)
\(concentration =\frac{number \ of \ moles}{volume}\)
\([Na_2S_2O_3]= \frac{9.99× 10^{-4}\ mol\ \ dm^{-3}}{0.250 \ dm^3} =4.00 × 10^{-3} mol\ \ dm^{-3}\)
- Concentration of iodine
- The titration of 10cm3 of iodine required an average of 15cm3 of sodium thiosulfate
- The molar ratio of sodium thiosulfate to iodine is 2-1 respectively
\(n(Na_2S_2O_3) = 4.00 × 10^{-3} mol\ \ dm^{-3} × 0.0150dm^3 = 6.00 × 10^{-5} mol\)
\(n(I_2)=\frac{6.00×10^{-5}\ mol }{2.00} =3.00 × 10^{-5}\ mol \)
\([I_2]=\frac{3.00× 10^{-5}mol}{0.01dm^3} =3.00× 10^{-3} mol \ dm^{-3}\)
Therefore in the calculations below 3.00 × 10-3 mol dm-3 will be used as the iodine concentration.
Roow data
- The standard deviation calculations have been calculated using the TI - 84 Plus CE calculator
Figure 10 - Table On The Volume (cm3± 0.05cm3) of 0.003 mol dm-3 Iodine Solution Used to Titrate 0.01 mol dm-3Ascorbic Acid Solution Until End Point After exposed to UV Light For 0, 900, 1800, 2700, 3600 seconds (±1s) (cm3)
Outliers
Refer to Appendix B for outlier calculations. The calculation of outliers for the volume of 0.003 mol dm-3 iodine solution used to titrate ascorbic acid solution until the end point after being exposed to −3 UV light for 0, 900, 1800, 2700, and 3600 seconds. The formula below has been applied:
Q1 = first lower quartile
Q3 = third upper quartile
IQR = Q3 − Q1
Lower Fence = Q1 − (1. 5 × IQR)
Upper Fence = Q3 + (1. 5 × IQR)
As seen from Appendix B indicates that there were no outliers for each volume of iodine solution used for different ascorbic acid solutions exposed for different lengths of time in all 5 trials. Therefore all data sets from each trial will be included in the processed data calculations.
The Average Volume of 0.003 moldm-3 Iodine Solution Used Calculations
\(\text{Average Volume of Iodine Solution Used}=\frac{\sum0.03\ mol\ dm^3\ of \ iodine\ solution\ used \ (cm^3)}{number\ of \ trials}\)
Figure 11 - Table On Average Volume (cm3 ± 0.05cm3) of 0.003 mol dm-3Iodine Solution Used to Titrate 0.01 mol dm-3 Ascorbic Acid Solution Until End Point After Exposed to UV Light For 0, 900, 1800, 2700, 3600 seconds (±1s) (cm3)
The Number of Moles of 0. 003 mol dm-3 Iodine Solution Reacted Calculations -
Figure 12 - Table On The Number Of Moles Of 0. 003 mol dm-3 Iodine Solution Reacted Calculations
The Concentration of 5cm3 Ascorbic Acid Calculations
● The molar ratio of iodine to ascorbic acid is 1-1 so the number of moles of iodine is equal to ascorbic acid
Figure 13 - Table On The Concentration Of 0. 003 mol dm Iodine Solution Reacted (cm3)
Figure 14 - The Length of Time of UV Light Exposure (0, 900, 1800, 2700, 3600 seconds) Against the Concentration of Ascorbic Acid Solution (mol dm-3)
- The graph above shows a negative gradient of the trendline, therefore the concentration of ascorbic acid is observed to decrease over the length of exposure to UV light.
Pearson correlation coefficient
The statistical analysis test Pearson correlation coefficient was selected, to indicate the strength of correlation between the length of time exposure to UV light (0, 900, 1800, 2700, 3600 seconds) and the concentration of ascorbic acid solution. All calculations are done with the TI-84 Plus CE calculator.
Figure 14 - Pearson Correlation Formula (Patil, 2023)
\(r=\frac{\sum(x_i-\overline{x})\ (y_i-\overline{y})}{\sqrt{\sum(x_i- \overline{x})^2\sum(y_i-\overline{y})^2}}\)
Where,
r = pearson Correlation Coefficient
\(x_i= × \ \ variable \ \ samples\)
\(\overline{x}= mean \ \ of \ \ values \ \ in × variable\)
\(y_i= y \ \ variable \ sample\)
\(\overline {y}= mean \ \ of \ \ values \ \ in \ \ y \ \ variable \)
Refer to Appendix C for the calculation of the Pearson correlation coefficient using TI-84 Plus CE. The r-value (Pearson correlation coefficient) was rounded to 3 significant figures showing an r-value of - 0.996. The correlation between the length of time exposure to UV light (0, 900, 1800, 2700, 3600 seconds) and the concentration of ascorbic acid solution was strongly negative.
Uncertainties calculation
- The percentage uncertainty of iodine titration calculation The formula used to calculate the percentage uncertainty of the volume of iodine solution used to reach the end point of titration -
\(\text{Percentage uncertainty} =\frac{\sum\frac{uncertainty\ of\ burette}{volume \ of\ iodine}}{number\ of\ trials}×\ 100\%\)
Percentage uncertainty of experimental steps
\(\text{Percentage uncertainty}=\frac{uncertainty\ of\ apparatus}{valu \ measure \ by\ apparatus}× 100\%\)
uncertainty of the electronic weighing scale mass of soluble starch + uncertainty of the measuring cylinder volume of water
- Total Percentage Uncertainty This can be calculated by summing up the uncertainty of the volume of iodine solution and uncertainties in Table 8 for each sample.
The absolute uncertainty of the ascorbic acid concentration can be calculated with the formula -
Absolute uncertainty = concentration of ascorbic acid × total % uncertainty
Conclusion
The experiment was designed to investigate the effect of exposure to UV light on the concentration of ascorbic acid (vitamin C). Through the iodine titration experiment, the investigation of the relationship between exposure to UV light and the concentration of ascorbic acid was possible through calculating the final concentration of ascorbic acid. The results have shown that as the length of UV light exposure increases, the concentration of ascorbic acid decreases. This is due to UV light breaking down ascorbic acid via photodegradation. All samples exposed to UV light (0.009084, 0.008508, 0.007704 and 0.006936 moldm -3) of 900, 1800, 2700 and 3600 seconds respectively had lower concentrations than the control group (0 seconds) of 0.009612 moldm−3 .
According to Graph 1, the trendline shows a negative trendline over the increased time of UV light exposure. This can be predicted that future values of increasing UV light exposure will result in a decrease in the concentration of ascorbic acid. The Pearson correlation coefficient further strengthens the reliability of the results. The r-value was -0.996, the value is close to -1 indicating a strong negative correlation of the data. Shows that the relationship as the time of exposure to UV light increases, the concentration of ascorbic acid will decrease. Therefore, it is indicated that UV light exposure enhances the rate of degradation of ascorbic acid (vitamin C)
The random error of the experiment was ± 0. 01, which is relatively small indicating a minimal error on the results of the data collected. An error that cannot be accounted for is human error, which is present when monitoring the end-point of the iodine titration by eye. The exact moment of the endpoint of the titration might differ from the other samples due to the change in colour of the ascorbic acid solution.
The use of ascorbic acid (vitamin C) is widely used in food production and skin care. Vitamin C cannot be naturally produced in the human body. Therefore, the results suggest that the packaging of vitamin C is crucial to retain the most nutrients. An opaque bottle that is laminated can decrease sunlight penetration to the solution, the users should avoid exposing vitamin C products to direct sunlight.
Evaluation
| Strengths | Impacts |
|---|---|
| Same ascorbic acid solution made for different trials | The same ascorbic acid solution was made for each trial of 0, 900, 1800, 2700, and 3600 seconds giving constant reliable concentration for comparison for the end result. The titration would have an initial same concentration and provide accurate results of the difference of concentration after UV exposure. |
| Preliminary test for experiment | A preliminary test was conducted to determine the rate of change of ascorbic acid concentration after UV exposure over a length of time. The UV exposure time length was then determined to design the experiment, allowing the student to measure the rate of photodegradation of ascorbic acid. |
| Carefully planned controlled variables | The controlled variables were carefully planned as it has the potential to distort the final results. The significance of the controlled variables can be observed through the low absolute uncertainty of the concentration of ascorbic acid. This indicates that the random error due to apparatus uncertainty is low. |
| No outliers | The presence of no outliers indicates the experiment procedure conducted by the student is constant and accurate. The data produced by the student is reliable and has the ability to conduct a titration experiment accurately |
| Low Standard Deviation | The standard deviation of the data is relatively low, indicating the data points are close to the mean. This indicates that the data collected for each trial minimised the risk of significant errors in the estimation of the data. |
Figure 19 - Table On Evaluation
| Weakness | Impacts |
|---|---|
| Time of conduction of titration | The samples of ascorbic acid could not be titrated at the same time after exposure to UV light. This may cause the concentration of ascorbic acid to decrease due to the oxidative nature of ascorbic acid. This is observed for the control group 0 seconds that the initial concentration of ascorbic acid prepared was 0.01mol dm−3 but the concentration of ascorbic acid indicated by iodine titration was 0.009612mol dm-3. |
| Light intensity and temperature of the laboratory | The light intensity and temperature of the lab were not controlled. This may have affected the rate of degradation of ascorbic acid. This could be improved by conducting the experiment in a laboratory with controlled light intensity and constant temperature. |
| Judging the end point of titration | The end point of titration was indicated by a uniform permanent dark blue hue judged by the eye. The colour of the end point of titration may vary from different samples and, therefore, may have affected the final concentration of ascorbic acid. |
Figure 20 - Table On
Extensions
Possible extensions to achieve more accurate and precise data for the investigation above. The student could conduct the experiment in a constant and controlled laboratory with constant temperature and light intensity. High-quality apparatus with minimal uncertainty could be used to reduce systematic error. Especially for the set-up of the UV light apparatus, an improvement that could be made is to set up the UV light source carried out in a completely dark room with boarded windows. The UV light source should also be contained in a closed radiation box to minimise the environmental effects and harm towards the human body. Possible extensions of the investigation include testing the method of different packaging effectiveness in preventing photodegradation of ascorbic acid. The samples of ascorbic acid can be exposed to UV light through a filter of the different types of packaging such as glass with different levels of UV coating and matte. Different types of ascorbic acid skin care serums can also be tested for their effectiveness in packaging against the concentration of ascorbic acid in the serum. This can be conducted with the same procedure but the length of exposure to UV light and intensity of the UV light may require preliminary testing to determine its value.
Bibliography
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Appendix
Appendix A
The Volumes (cm3 ± 0.05) of 0.003 mol dm −3 Iodine Solution Used to Titrate Ascorbic Acid Solution Until the End Point After being Exposed to UV Light For 0 seconds (cm3)
Figure 21 - Table On The Volume (cm3 ± 0.05) of 0.003 mol dm−3 Iodine Solution Used to Titrate Ascorbic Acid Solution Until End Point After Exposed to UV Light For 0 seconds (cm3)
Figure 22 - Table On The Volume (cm3 ± 0.05) of 0.003 mol dm−3 Iodine Solution Used to Titrate Ascorbic Acid Solution Until End Point After Exposed to UV Light For 900 seconds (cm3)
Figure 23 - Table On The Volume (cm3 ± 0.05) of 0.003 mol dm−3 Iodine Solution Used to Titrate Ascorbic Acid Solution Until End Point After Exposed to UV Light For 1800 seconds (cm3)
Figure 24 - Table On The Volume (cm3 ± 0.05) of 0.003 mol dm−3 Iodine Solution Used to Titrate Ascorbic Acid Solution Until End Point After Exposed to UV Light For 2700 seconds (cm3)
Figure 25 - Table On The Volume (cm3 ± 0.05) of 0.003 mol dm−3 Iodine Solution Used to Titrate Ascorbic Acid Solution Until End Point After Exposed to UV Light For 3600 seconds (cm3)
Appendix B
Appendix C
Pearson Correlation Coef icient Between The Length of Time Exposure to UV Light (± 1s) (0, 15, 30, 45, 60 min) and the Concentration of Ascorbic Acid Solution (mol dm-3)
